A survey of phenytoin urines will continue with quantitation of para- and meta-hydroxylated metabolites and of the dihydrodiol metabolite in an effort to determine if metabolic patterns can be correlated with the incidence of adverse reactions. Urines from patients receiving mephenytoin will be studied. Metabolites to be measured quantitatively are the demethylated and the para-hydroxylated metabolites and the metabolite that is both demethylated and hydroxylated. These metabolites will also be isolated and their enantiomeric compositions determined. The absolute configurations of the optical isomers of 5-(3-hydroxyphenyl)-5-phenylhydantoin and 5-ethyl-5-(4-hydroxyphenyl)hydantoin, metabolites of phenytoin and mephenytoin, respectively, will be determined by single crystal X-ray diffraction methods.